This invention relates to technology for inspection reticles used in manufacturing semiconductor wafers and more particularly for inspecting sub-resolution assist features (SRAF) on a reticle.
An integrated circuit (IC) is a small electronic circuit consisting mainly of semiconductor devices that has been manufactured on the surface of a thin substrate of semiconductor material, such as silicon wafers. A typical manufacturing process involves marking different areas of the wafers to be doped or to have polysilicon, insulators or conductive metal (copper, aluminum, etc.) deposited on them. The process is generally known as photolithography. During a photolithography process, light is typically used to transfer a geometric patterns representing various features of the integrated circuit from a reticle to a light-sensitive photoresist deposited on a wafer. The subsequent chemical treatment then engraves the exposed patterns into the material underneath the photoresist. The cycle is repeated for each new layer of the IC.
A typical reticle is a quartz plate that contains 4×, 5×, 10× or other magnification patterns to be reproduced on a wafer within a field area that encompasses a single die or several dies. A reticle is also called a mask or a photomask. Allowing for reduction in size of the pattern from a reticle to a wafer eases the burden of producing an acceptable reticle. During exposure the light passes only through the portions of the reticle that do not contain any images, e.g., transparent, and exposing the photoresist on the wafer. The exposed photoresist has different chemical resistant properties than unexposed one. There are two types of the photoresist: positive and negative. More common positive photoresist becomes chemically less stable when exposed and will be etched away during the subsequent process. Negative photoresist becomes more stable and unexposed parts of the photoresists are etched away. Therefore, using negative photoresist results in features on a wafer corresponding to images on a reticle, while using positive photoresist results in features on a wafer corresponding to gaps between images on a reticle. In either case, transmitted light is further passed through a reduction lens that reduces the size of the image. The reduced image is then projected to a selected field on the wafer, where the field position is determined by a device known as a stepper. The photoresist on the wafer is exposed at this field position.
Correlation between reticle's pattern and IC's layout lead to optical inspection of reticles that has become a standard procedure in the production of IC's. During one type of conventional inspection process, an optical image of the reticle is typically compared to a baseline image. The baseline image is either generated from the circuit pattern data or from an adjacent die on the reticle itself. Either way, the optical image features are analyzed and compared with corresponding features of the baseline image. Each feature difference is then compared against a single threshold value. If an optical image feature varies from the corresponding baseline feature by more than the predetermined threshold, a defect is defined. Although these conventional reticle inspection methods provide adequate levels of detection accuracy for some applications, such inspections have been found to be inadequate for certain reticle features, such as subresolution assist features (or SRAF's). For example, when a conventional inspection is applied to all features, including SRAF's, the inspection may detect numerous nuisance defects that would not result in a defective IC and/or fail to capture “real” defects that would result in a defective IC.
Accordingly, it would be beneficial to provide improved inspection techniques for detecting real defects, such as defective SRAF's, on a reticle while minimizing the detection of nuisance defects.